Problem: In math class, the girl to boy ratio is $4$ to $5$. If there are a total of $45$ students, how many girls are there?
Answer: A ratio of $4$ girls to $5$ boys means that a set of $9$ students will have $4$ girls and $5$ boys. A class of $45$ students has $5$ sets of $9$ students. Because we know that there are $4$ girls in each set of $9$ students, the class must have $5$ groups of $4$ girls each. There is a total of $20$ girls in math class.